Osteoporosis, with its increased fracture risk, is a major public health concern. While the majority of past prevention efforts have targeted slowing bone loss, there is great interest in skeletal adaptations to interventions during development. Both bone mass and structural geometry are important determinants of bone strength and fracture risk. Animal studies clearly show structural adaptations to mechanical stimuli during development. Children and adolescents, more so than adults, may have the ability to modify structural geometry as well as increase bone mineral mass and density in response to increased loads, which would increase bone strength and potentially protect against fractures later in life. This proposition has not been adequately tested. Moreover, from the few available studies, there have been conflicting reports regarding relative adaptations in girls at different stages of development. Thus the optimal timing of interventions is not known. To address these issues, we propose to assess the effects of "ground-reaction (e.g., jumping, hopping, etc.) exercise on bone macro-architectural development in pre-pubescent and early pubescent girls. Bone mineral content (BMC), bone mineral density (BMD), and bone structural geometry will be measured in girls in 4th (Tanner stage 1) and 6th (Tanner stage II and III) grade at baseline, and at regular intervals thereafter, to assess the effects of two years of exercise on bone accrual and structure. Follow-up measures will be made at 1 and 2 years post intervention to assess whether intervention effects persist. Physical activity, diet, maturation, body size and body composition will also be measured to control for differences in growth, maturation, and important environmental factors that may influence bone development and the interpretation of the adaptations to the intervention. This study is unique in several ways including: i) its focus on structural changes in addition to BMC and BMD;ii) the focus on pre- and early pubescent girls to define the optimal "window of opportunity" for adaptation to occur;iii) the two-year intervention so we can examine whether benefits continue to accrue with longer duration intervention;iv) comprehensive measurement of important covariates;and v) its focus on the maintenance of adaptations over two years of follow-up after intervention. Past studies have failed to adequately control for body and bone size, which confound area) BMD from DXA, and which vary with maturation. The inclusion of pQCT vBMD and measures of structural geometry allows an appropriate interpretation of changes in BMC and BMD, and provides a prospective test of the effects of exercise on structural adaptations in developing girls which contribute to bone strength and which may have lasting benefit for lowering fracture risk. We expect this study will make a significant contribution to our understanding of bone macro-architectural adaptations to exercise and will provide critical information for the design and optimal timing of interventions designed to enhance bone development to lessen future fracture risk.